you can make it look like the stage is 500x281 but you cannot actually change the stage size unless you publish for 100%, use a scalemode of noscale and use javascript to resize the browser window (which you can only do if the window was opened using javascript).

No I have a 1920x1080 stage that needs to fit in a 500x281 place on a webpage. The problem is that when I go fullscreen the screen is bigger than the moniter and when I exit "normal" == 1920x1080. (I need "normal" to == 500x281 and "fullscreen" == 100% width and 100% height) So the animation is bigger than 500x281, so it looks cropped